Floating Production Storage and Offloading (FPSO) ships contain processes designed to process hydrocarbon streams from subsea wells to produce oil. The oil is typically stored on the ship until it can be offloaded onto tankers and further processed on land. A byproduct of oil production is the formation of a natural gas, which can be treated to remove contaminants and produce treated natural gas that is utilized onshore in typical natural gas pipelines. Prior to entering the pipeline, however, the natural gas must be processed to meet the pipeline specifications. In particular, pipelines generally require a specific purity of the treated natural gas with respect to various contaminants including carbon dioxide, water, and sulfur so as to not damage the pipeline.
The use of solvent-based systems to remove contaminants from natural gas is well known and utilized in prior art systems. However, there are various drawbacks to solvent-based purification systems. In particular, solvent-based systems suffer from various operating concerns due to rocking motion on a floating vessel such as a ship, and the liquid nature of the solvent. The rocking motion inside the various operating units may result in low adsorption efficiency, which may be insufficient for the treated natural gas to meet the specifications for the pipeline. There are further concerns regarding health, safety, and the environment with the use of a liquid solvent on a floating vessel. Further, the operating units of solvent-based systems are typically larger and require more space, which is frequently limited by the vessel size. Additionally, solvent-based systems typically include more restrictive operating parameters because the solvent must be kept circulating though the system and the solvent flow rate is difficult to adjust.
This invention overcomes various problems associated with solvent-based systems, either on a vessel or on land. In particular, the process of the present invention allows for a membrane and molecular sieve treatment process that is not solvent-based. The process disclosed herein reduces the health, safety, and environmental concerns of a solvent-based system because of the absence of a toxic liquid solvent on an unstable vessel. Further, the dry process disclosed herein includes an increased purity of the treated natural gas due to the process functioning consistently independent of any rocking motion. The process further includes a compact, modular construction that allows for increased flexibility. Unlike the solvent-based systems that typically utilize larger unwieldy distillation columns, compact and modular construction of the components used in this process is possible because the membrane and sieve units may be built in groups in discrete sections and stacked in an efficient manner. Finally, operating efficiencies are realized because the modular components may be operated independently from one another, which allows the process to perform effectively throughout a large operating envelop including turndown conditions.
The process of the present invention is designed to produce a treated natural gas having the requisite purity to be sent through a pipeline. The process may take place on an ocean vessel, such as a ship, or a land based facility. The process includes a cleaning step having a membrane unit that can achieve the bulk removal of contaminants such as carbon dioxide, water, and hydrogen sulfide followed by a polishing step that includes a molecular sieve unit specially provided to minimize the hydrogen sulfide content of the treated natural gas. The removal of hydrogen sulfide from the treated natural gas is important to prevent corrosion in the pipeline.